In the combustion of a fuel, such as coal, oil, natural gas, peat, waste, etc., in a combustion plant, such as a power plant or a waste incineration plant, a process gas is generated. For separating nitrogen oxides, usually denoted NOx, from such a process gas, often referred to as a flue gas, a method is frequently used, in which a reducing agent, usually ammonia or urea, is mixed with the flue gas. The flue gas, mixed with said ammonia or urea, is then passed through a catalyst in which the reducing agent reacts selectively with the NOx to form nitrogen gas and water vapour. Usually the catalyst is installed in a so called Selective Catalytic Reduction reactor (SCR reactor).
In many processes, the concentration of NOx of the flue gas is not evenly distributed over the cross-section of the SCR reactor. This poses a problem, since a stoichiometric ratio between the NOx and the reducing agent is essential for achieving a good reduction of the NOx content of the flue gas and a low slip of the reducing agent from the SCR reactor.
U.S. Pat. No. 6,905,658 discloses an SCR reactor in which a gas flow is divided into several separate flow channels upstream of the catalytic material. Each flow channel is provided with a separate means for supplying ammonia to that specific flow channel. By individually adjusting each of said separate means for supplying ammonia it becomes possible to achieve a close to stoichiometric ratio between the NOx and the ammonia.
A problem of U.S. Pat. No. 6,905,658 is that a large number of separate means for supplying ammonia that are individually controllable increases the cost of the SCR reactor.